Advanced polymer/wood composite pellet process

ABSTRACT

A process for making a polyvinyl chloride and wood composite pellet by extrusion. The polyvinyl chloride and wood fiber are introduced into the extruder at a ratio of approximately 6:4 by weight, respectively. The polyvinyl chloride is melted and blended with the wood fiber so that the interstitial voids of the wood fiber are filled with polyvinyl chloride. After being extruded through a die, the molten material is cut into pellets having a bulk density of preferably between 0.7 to 0.8 gm/mm 3 .

This is a DIVISION of application Ser. No. 08/017,240, filed Feb. 12,1993, now Deaner et al, U.S. Pat. No. 5,441,801 which application isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention pertains to an extrusion process for making anadvanced polymer/wood composite pellet.

BACKGROUND OF THE INVENTION

Conventional window and door manufacture has commonly used wood andmetal components in forming structural members. Residential windows arecommonly manufactured from milled wood products that are assembled withglass to form double hung or casement units. Wood windows whilestructurally sound, useful and well adapted for use in many residentialinstallations, can deteriorate under certain circumstances. Wood windowsalso require painting and other periodic maintenance. Wood windows alsosuffer from cost problems related to the availability of suitable woodfor construction. Clear wood products are slowly becoming more scarceand are becoming more expensive as demand increases. Metal componentsare often combined with glass and formed into single unit slidingwindows. Metal windows typically suffer from substantial energy lossduring winter months.

Extruded thermoplastic materials have been used in window and doormanufacture. Filled and unfilled thermoplastics have been extruded intouseful seals, trim, weatherstripping, coatings, and other windowconstruction components. Thermoplastic materials, such as polyvinylchloride, have been combined with wood members in manufacturingPERMASHIELD® brand windows manufactured by Andersen Corporation for manyyears. The technology disclosed in Zanini, U.S. Pat. Nos. 2,926,729 and3,432,883, have been utilized in the manufacturing of plastic coatingsor envelopes on wooden or other structural members. Generally, thecladding or coating technology used in making PERMASHIELD® windowsinvolves extruding a thin polyvinyl chloride coating or envelopesurrounding a wooden structural member.

Polymer materials have combined with cellulosic fiber to make extrudedmaterials. However, such materials have not successfully been used inthe form of a structural member that is a direct replacement for wood.Common extruded thermoplastic composite materials cannot provide thermaland structural properties similar to wood or other structural materials.These extruded materials fail to have sufficient modulus, compressivestrength, coefficient of thermal expansion that matches wood to producea direct replacement material. Further, many prior art extrudedcomposites must be milled after extrusion to a final useful shape.Typical commodity plastics have achieved a modulus no greater than about500,000. One class of composite, a polyvinyl chloride/wood flourmaterial, poses the added problem that wood dust, which can accumulateduring manufacture, tends to be explosive at certain concentrations ofwood flour in the air. Most commonly, polyvinyl chloride, polystyrene,and polyethylene thermoplastics have been used in such products.

Accordingly, a substantial need exists for a composite material that canbe made of polymer and wood fiber with an optional, intentional recycleof a waste stream. A further need exists for a composite material thatcan be extruded into a shape that is a direct substitute for theequivalent milled shape in a wooden or metal structural member. Thisneed requires a coefficient of thermal expansion that approximates wood,a material that can be extruded into reproducible stable dimensions, ahigh compressive strength, a low thermal transmission rate, an improvedresistance to insect attack and rot while in use, and a hardness andrigidity that permits sawing, milling, and fastening retentioncomparable to wood members.

Further, companies manufacturing window and door products have becomesignificantly sensitive to waste streams produced in the manufacture ofsuch products. Substantial quantities of wood waste including wood trimpieces, sawdust, wood milling by-products, recycled thermoplasticincluding recycled polyvinyl chloride, have caused significant expenseto window manufacturers. Commonly, these materials are either burned,for their heat value in electrical generation, or are shipped toqualified landfills for disposal. Such waste streams are contaminatedwith substantial proportions of hot melt and solvent-based adhesives,waste thermoplastic such as polyvinyl chloride, paint, preservatives,and other organic materials. A substantial need exists to find aproductive, environmentally-compatible process for using such wastestreams for useful structural members and, thus, to avoid returning thematerials into the environment in an environmentally harmful way.

SUMMARY OF THE INVENTION

This invention pertains to a process of combining cellulosic fiber andthermoplastic polymer materials, for example, wood fiber and polyvinylchloride to form a pellet of composite material to be later used informing structural members. The composite material can be used to formstructural members instead of wood, polyvinyl chloride, or metal such asaluminum.

The process of this invention manufactures a pellet composite comprisinga thermoplastic polymer and a cellulosic fiber. In making the pellet,about 35 to 50 parts of fiber and about 45 to 70 parts of polymer pereach 100 parts of the composite are combined in an extruder. Thematerials are blended under melt conditions in the extruder undersufficient conditions of temperature and pressure to cause the polymericcomposition to mix with the fiber. Mixing is continued to the extentthat the cells of the fiber are disrupted. The thermoplastic polymer(s)are introduced into the interior volume of the cell causing asubstantial increase in the bulk density of the polymer material. Thecomposite mixture contains thermoplastic polymer which has beenintroduced into the interior volume of the cells such that greater than50% of the cellular interior volume is occupied by polymer. The hot meltcomposite is extruded from the extruder equipment. We have found thatusing the process of this invention a pellet can be made wherein theratio of (i) the density of a simple physical mixture of the fiber, withundisrupted cells, and polymer to (ii) the density of the pellet withpolymer introduced into the interior volume of the cell, is less than0.8, preferably less than 0.7.

The preferred composite material of this invention can be made from anypolyolefin, polystyrene, polyacrylic or polyester. The most preferredsystem is polyvinyl chloride and wood fiber wherein the density of thepellet is greater than about 0.6 gram per cm³. Preferably the density ofthe pellet is greater than 0.7 gram per cm³ and for reasons of improvedthermal properties, structural properties, modulus compression strength,etc., the bulk density of the pellet is greater than 0.8 gram per cm³.

We have found that the temperature profile of the extruder is importantin developing the improved physical properties of the invention. We havefound that having a high initial temperature in the input zone of theextruder is important while reducing the temperature through theextruder zones as the material passes from the input zone throughinterior zones to the die zone aids in rupturing fiber cell wall andintroducing the polymer into the interior volume of the cell.Accordingly, the temperature of the input zone is substantially higherthan the melt point of the polymer, the temperature of the interiorzones of the extruder are maintained higher than the melt point of thepolymer but less than the temperature of the input zone. In a preferredmode of the invention, the temperature profile of the extruder comprisesa high input temperature, a step-wise reduction in temperature as thematerial passes through the interior zones of the extruder but thetemperature of the pellet die zone is greater than the interior zone butless than the input zone. In the most preferred pellet compositions ofthe invention, the polyvinyl chloride occupies greater than 65% of theinterior volume of the wood fiber cell and most preferably greater than70% of the interior volume of the wood fiber cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a box diagram of an extruder used to perform the process ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention relates to a polymer/wood composite pellet process forcombining polyvinyl chloride and wood fiber to form a compositepelletized material having a controlled water content. The processgenerally involves combining and blending polyvinyl chloride and woodfiber by thermoplastic extrusion so that intimate contact between thepolyvinyl chloride and wood fiber leads to thorough wetting and fillingof the interstitial voids of the wood fiber by the polyvinyl chloride toensure high quality physical properties in the extruded compositematerial. The pelletized material can subsequently be remelted andextruded to form structural members for use as a substitute for wood,aluminum or other materials.

Thermoplastic Polymers

The pellet of the invention comprises a composite made by combining athermoplastic polymer with a cellulosic fiber under conditions of hightemperature, pressure and shear. The pellet attains a surprisingly highdensity when compared to prior art materials resulting in improvedphysical and structural properties.

Thermoplastic polymers that can be used in the invention comprise wellknown classes of thermoplastic polymers including polyolefins such aspolyethylene, polypropylene, poly(ethylene-copropylene),polyethylene-co-alphaolefin) and others. Polystyrene polymers can beused including polystyrene homopolymers, polystyrene copolymers andterpolymers; polyesters including polyethylene terephthalate,polybutylene terephthalate, etc. and halogenated polymers such aspolyvinyl chloride, polyvinylidene chloride and others.

The pellet of the invention uses a cellulosic fiber. The cellulosicfiber commonly comprises fibers having a high aspect ratio made of cellswith cellulosic cell walls. During the process of the invention, thecell walls are disrupted and polymers introduced into the interior voidvolume of the cells under conditions of high temperature and pressure.The cellulosic material can be derived from a variety of sourcesincluding hard and soft wood products and by-products, sugar cane,cotton, flax and other known sources of cellulosic materials. Thepreferred source of cellulosic fiber for this invention comprises woodfiber which can be product or by-product of the manufacture of lumber orother wood products.

Polyvinyl Chloride Homopolymer, Copolymers and Polymeric Alloys

Polyvinyl chloride is a common commodity thermoplastic polymer. Vinylchloride monomer is made from a variety of different processes such asthe reaction of acetylene and hydrogen chloride and the directchlorination of ethylene. Polyvinyl chloride is typically manufacturedby the free radical polymerization of vinyl chloride resulting in auseful thermoplastic polymer. After polymerization, polyvinyl chlorideis commonly combined with thermal stabilizers, lubricants, plasticizers,organic and inorganic pigments, fillers, biocides, processing aids,flame retardants, and other commonly available additive materials.Polyvinyl chloride can also be combined with other vinyl monomers in themanufacture of polyvinyl chloride copolymers. Such copolymers can belinear copolymers, branched copolymers, graft copolymers, randomcopolymers, regular repeating copolymers, block copolymers, etc.Monomers that can be combined with vinyl chloride to form vinyl chloridecopolymers include an acrylonitrile; alpha-olefins such as ethylene,propylene, etc.; chlorinated monomers such as Vinylidene dichloride;acrylate monomers such as acrylic acid, methylacrylate,methylmethacrylate, acrylamide, hydroxyethyl acrylate, and others;styrenic monomers such as styrene, alphamethyl styrene, vinyl toluene,etc.; vinyl acetate; and other commonly available ethylenicallyunsaturated monomer compositions.

Such monomers can be used in an amount of up to about 50 mol-%, thebalance being vinyl chloride. Polymer blends or polymer alloys can beused in the pellet process of this invention. Such alloys typicallycomprise two miscible polymers blended to form a uniform composition.Scientific and commercial progress in the area of polymer blends haslead to the realization that important physical property improvementscan be made not by developing new polymer material but by formingmiscible polymer blends or alloys. A polymer alloy at equilibriumcomprises a mixture of two amorphous polymers existing as a single phaseof intimately mixed segments of the two macro molecular components.Miscible amorphous polymers form glasses upon sufficient cooling and ahomogeneous or miscible polymer blend exhibits a single,composition-dependent glass transition temperature (T_(g)). Immiscibleor non-alloyed blend of polymers typically displays two or more glasstransition temperatures associated with immiscible polymer phases. Inthe simplest cases, the properties of polymer alloys reflect acomposition weighted average of properties possessed by the components.In general, however, the property dependence on composition varies in acomplex way with a particular property, the nature of the components(glassy, rubbery or semi-crystalline), the thermodynamic state of theblend, and its mechanical state whether molecules and phases areoriented. Polyvinyl chloride forms a number of known polymer alloysincluding, for example, polyvinyl chloride/nitrile rubber; polyvinylchloride and related chlorinated copolymers and terpolymers of polyvinylchloride or vinylidine dichloride; polyvinyl chloride/alphamethylstyrene-acrylonitrile copolymer blends; polyvinyl chloride/polyethylene;polyvinyl chloride/chlorinated polyethylene; and others.

The primary requirement for the substantially thermoplastic polymericmaterial is that it retain sufficient thermoplastic properties to permitmelt blending with wood fiber, permit formation of linear extrudatepellets, and to permit the composition material or pellet to be extrudedor injection molded in a thermoplastic process forming a rigidstructural member. Polyvinyl chloride homopolymers copolymers andpolymer alloys are available from a number of manufacturers including B.F. Goodrich, Vista, Air Products, Occidental Chemicals, etc. Preferredpolyvinyl chloride materials are polyvinyl chloride homopolymer having amolecular weight of about 90,000±50,000, most preferably about88,000±10,000. The preferred polyvinyl chloride has a bulk density ofapproximately 0.71 gm/cc±0.10 gm/cc.

Wood Fiber

Wood fiber, in terms of abundance and suitability, can be derived fromeither soft woods or evergreens or from hard woods commonly known asbroad leaf deciduous trees. Soft woods are generally preferred for fibermanufacture because the resulting fibers are longer, contain highpercentages of lignin and lower percentages of hemicellulose than hardwoods. While soft wood is the primary source of fiber for the invention,additional fiber make-up can be derived from a number of secondary orfiber reclaim sources including bamboo, rice, sugar cane., and recycledfibers from newspapers, boxes, computer printouts, etc.

However, the primary source for wood fiber used in the process of thisinvention comprises the wood fiber by-product of sawing or milling softwoods commonly known as sawdust or milling tailings. Such wood fiber hasa regular reproducible shape and aspect ratio. The fibers based on arandom selection of about 100 fibers are commonly at least 3 mm inlength, 1 mm in thickness and commonly have an aspect ratio of at least1.8. Preferably, the fibers are 1 to 10 mm in length, 0.3 to 1.5 mm inthickness with an aspect ratio between 2 and 7, preferably 2.5 to 6.0.

The preferred fiber for use in this invention are fibers derived fromprocesses common in the manufacture of windows and doors. Wooden membersare commonly ripped or sawed to size in a cross grain direction to formappropriate lengths and widths of wood materials. The by-product of suchsawing operations is a substantial quantity of sawdust. In shaping aregular shaped piece of wood into a useful milled shape, wood iscommonly passed through machines which selectively remove wood from thepiece leaving the useful shape. Such milling operations producesubstantial quantities of sawdust or mill tailing by-products. Lastly,when shaped materials are cut to size and mitered joints, butt joints,overlapping joints, mortise and tenon joints are manufactured frompre-shaped wooden members, substantial waste trim is produced. Suchlarge trim pieces are commonly cut and machined to convert the largerobjects into wood fiber having dimensions approximating sawdust or milltailing dimensions. The wood fiber sources of the invention can beblended regardless of particle size and used to make the composite. Thefiber stream can be pre-sized to a preferred range or can be sized afterblending. Further, the fiber can be pre-pelletized before use incomposite manufacture.

Such sawdust material can contain substantial proportions of wastestream by-products. Such by-products include waste polyvinyl chloride orother polymer materials that have been used as coating, cladding orenvelope on wooden members; recycled structural members made fromthermoplastic materials; polymeric materials from coatings; adhesivecomponents in the form of hot melt adhesives, solvent based adhesives,powdered adhesives, etc.; paints including water based paints, alkydpaints, epoxy paints, etc.; preservatives, anti-fungal agents,anti-bacterial agents, insecticides, etc.; and other waste streamscommon in the manufacture of wooden doors and windows. The total wastestream content of the wood fiber materials is commonly less than 25 wt-%of the total wood fiber input into the polyvinyl chloride wood fiberproduct. Of the total waste recycle, approximately 10 wt-% of that cancomprise a vinyl polymer commonly polyvinyl chloride. Commonly, theintentional recycle ranges from about 1 to about 25 wt-%, preferablyabout 2 to about 20 wt-%, most commonly from about 3 to about 15 wt-% ofcontaminants based on the sawdust. The sawdust preferably has a densityof 0.15 gm/cc±0.30 gm/cc.

Extruder

The preferred extruder for performing the process of the presentinvention is a Cincinnati Milacron 80 (CM-80) Multiscrew Extruderreferred to generally by the numeral 10 in FIG. 1. Other extruders suchas the Moldovia 70 could also be used, but the CM-80 is the preferred.The CM-80 is a positive displacement pump for the controlled transportof material through a die. Major components of the unit are:

(a) a barrel 18 and two screws to transport, mix, and compress thematerial;

(b) a heating and cooling system to control the heat needed forprocessing;

(c) a drive train to power the screws; and

(d) a control station for controlling the operation of the extruder,including instruments to monitor the process and indicate possibleproblems.

Barrel 18 has a meshing, dual, conical taper bore. The combination ofbarrel taper and screw flight provides proper compression of thecomposite material. Barrel 18 is manufactured in three sections but hasfour operational zones.

The larger outside diameter of the screws in the barrel intake area zone1 allows for good intake of material and provides a large surface areafor heat transfer into the material. Between zones 2 and 3 is a vent toallow moisture and other gases to escape from barrel 18. Zone 4 is ametering zone where the material is compressed and metered out to a die22. Between die 22 and extruder barrel 18 is an adapter 20 having twozones, referred to in FIG. 1 as zones 5 and 6. Die 22 has one zone,referred to in FIG. 1 as zone 7.

The screws each have a hollow core through which heat transfer liquid iscirculated. This allows the transfer of unwanted frictional heat fromzone 4 back toward zone 1. The four heat zones on barrel 18 areindependently controlled by electric heat bands and heat exchangers foraccurate stabilization of operational temperatures. Likewise, zones 5and 6 on adapter 20 and zone 7 on die 22 are independently controlled byheater bands and heat exchangers for accurate temperature control. Thetemperature of the heat transfer liquid in the screws is alsoindependently controlled. The preferred screws are Cincinnati MilacronPelletizing Screws, the preferred die is a Cincinnati Pelletizing DieModel GK120.

Heat is applied to the four barrel zones by the electric heat bands.Heat is removed from barrel 18 by circulating oil through coils woundaround the barrels at zones 2, 3 and 4. The oil is circulated by a pumpunit through a heat exchanger located in the base of the extruder. Thebarrel cooling system is equipped with flow indicators for a visualcheck of the cooling performance in each cooling zone. Screw corecooling/heating is accomplished with an independent temperature controlunit.

The drive train begins with an infinitely variable speed, constanttorque drive motor coupled to a speed reducer. From the speed reducer,the drive train evolves into a two-shaft distribution gear drive. Sincethe drive shafts are on the center lines of their corresponding taperedscrews, it is possible to use large gears for torque transfer and largebearings to take up the axial force (back pressure) generated in theoperation of the extruder. In order to achieve consistent quality ofproduct, the speed and motor load of the drive train and the axial loadof the thrust bearings are measured.

Wood fiber is introduced into barrel 18 at zone 1 by gravity from a woodfiber hopper 12. The input of wood fiber is preferably metered by a feedscrew. The polyvinyl chloride is introduced into zone 1 of barrel 18 bygravity from a polyvinyl chloride hopper 14. The flow of polyvinylchloride from hopper 14 is preferably metered by a feed screw. From acarbon black hopper 16, carbon black can also be introduced into thewood fiber and polyvinyl chloride input stream. The carbon black ispreferably metered into the input stream by a feed screw. The rate ofwood fiber, polyvinyl chloride, and carbon black input to barrel 18 iscontrolled from the control panel.

Positioned at the distal end of die 22 is a cutter 24. Cutter 24preferably has two oppositely disposed cutting blades. The cuttingblades are operably connected to an axle so that with each rotation ofthe axle, each cutting blade will pass each opening of die 22. The timeperiod between each pass of the blade by a given opening in die 22should be approximately the same to achieve a constant pellet length.

Pellets

The process of this invention combines the polyvinyl chloride and woodfiber to form a pellet using thermoplastic extrusion. Wood fiber can beintroduced into pellet-making process in a number of sizes. We believethat the wood fiber should have a minimum size of length and width of atleast 1 mm because wood flour tends to be explosive at certainwood-to-air ratios. Further, wood fiber of appropriate size of a aspectratio greater than 1 tends to increase the physical properties of theextruded structural member. However, useful structural members can bemade with a fiber of very large size. Fibers that are up to 3 cm inlength and 0.5 cm in thickness can be used as input to the pellet orlinear extrudate manufacturing process. However, particles of this sizedo not produce highest quality structural members or maximizedstructural strength. The best appearing product with maximizedstructural properties are manufactured within a range of particle sizeas set forth below. Further, large particle wood fiber can be reduced insize by grinding or other similar processes that produce a fiber similarto sawdust having the stated dimensions and aspect ratio. One furtheradvantage of manufacturing sawdust of the desired size is that thematerial can be pre-dried before introduction into the pellet or linearextrudate manufacturing process. Further, the wood fiber can bepre-pelletized into pellets of wood fiber with small amounts of binderif necessary.

During the pelletizing process for the composite pellet, the polyvinylchloride and wood fiber are intimately contacted at high temperaturesand pressures to insure that the wood fiber and polymeric material arewetted, mixed and extruded in a form such that the polymer material, ona microscopic basis, coats and flows into the pores, cavity, etc.,including the interstitial voids of the fibers.

The fibers are preferably substantially oriented by the extrusionprocess in the extrusion direction. Such substantial orientation causesoverlapping of adjacent parallel fibers and polymeric coating of theoriented fibers resulting in a material useful for manufacture ofimproved structural members with improved physical properties. Thedegree of orientation is about 20%, preferably 30% above randomorientation, which is about 45 to 50%. The structural members havesubstantially increased strength and tensile modulus with a coefficientof thermal expansion and a modulus of elasticity that is optimized forwindow and doors. The properties are a useful compromise between wood,aluminum and neat polymer.

Moisture control is an important element of manufacturing a usefullinear extrudate or pellet. The concentration of water present in thesawdust during the formation of pellet or linear extrudate when heatedcan flash from the surface of the newly extruded structural member andcan come as a result of a rapid volatilization, form a steam bubble deepin the interior of the extruded PG,17 member which can pass from theinterior through the hot thermoplastic extrudate leaving a substantialflaw. In a similar fashion, surface water can bubble and leave cracks,bubbles or other surface flaws in the extruded member.

Trees when cut, depending on relative humidity and season, can containfrom 30 to 300 wt-% water based on fiber content. After rough cuttingand finishing into sized lumber, seasoned wood dan have a water contentof from 20 to 30 wt-% based on fiber content. Kiln-dried sized lumbercut to length can have a water content typically in the range of 8 to12%, commonly 8 to 10 wt-% based on fiber. Some wood source, such aspoplar or aspen, can have increased moisture content while some hardwoods can have reduced water content. Because of the variation in watercontent of wood fiber source and the sensitivity of extrudate to watercontent, control of water to a level of less than 8 wt-% in the pellet,based on pellet weight, is important.

The pellets made in accordance with the invention formed by extrusionthrough a die result in a linear extrudate that can be cut into a pelletshape. The pellet cross-section can be any arbitrary shape depending onthe extrusion die geometry. However, we have found that a regulargeometric cross-sectional shape can be useful. Such regularcross-sectional shapes include a triangle, a square, a rectangle, ahexagonal, an oval, a circle, etc. The preferred shape of the pellet isa regular cylinder having a roughly circular or somewhat ovalcross-section. The Cincinnati Pelletizing Die Model GK120 produces apellet having an approximately circular cross-section.

The preferred pellet is a right circular cylinder, the preferred radiusof the cylinder is at least 1.5 mm with a length of at least 1 mm.Preferably, the pellet has a radius of 1 to 5 mm and a length of 1 to 10mm. Most preferably, the cylinder has a radius of 2.3 to 2.6 mm, alength of 6.4 to 8.5 mm, and a bulk density of about 0.7 to 0.8 gm/mm³.

We have found that the interaction, on a microscopic level, between thepolymer mass and the wood fiber is an important element of theinvention. We have found that the physical properties of an extrudedmember are improved when the polymer melt during extrusion of the pelletor linear member thoroughly wets and penetrates the wood fiberparticles. The thermoplastic material comprises an exterior continuousorganic polymer phase with the wood particle dispersed as adiscontinuous phase in the continuous polymer phase. The material duringmixing and extrusion obtains an aspect ratio of at least 1.1 andpreferably between 2 and 4, optimizes orientation such as at least 20wt-%, preferably 30% of the fibers are oriented in an extruderdirection, and are thoroughly mixed and wetted by the polymer such thatall exterior surfaces of the wood fiber are in contact with the polymermaterial. This means that any pore, crevice, crack, passage way,indentation, etc., is fully filled by thermoplastic material. Suchpenetration is attained by ensuring that the viscosity of the polymermelt is reduced by operations at elevated temperature and the use ofsufficient pressure to force the polymer into the availableinterstitials voids and surface cracks and crevices of the wood fiber.

During the extrusion process, substantial work is done in providing auniform dispersion of the wood into the polymer material. Such workproduces substantial orientation which, when extruded into a finalstructural member, permits the orientation of the fibers in thestructural member to be increased in the extruder direction resulting inimproved structural properties.

The pellet dimensions are selected for both convenience in manufacturingand in optimizing the final properties of the extruded materials. Apellet with dimensions substantially less than the dimensions set forthabove is difficult to extrude, pelletize and handle in storage. Pelletslarger than the range recited are difficult to introduce into extrusionor injection molding equipment, and are difficult to melt and form intoa finished structural member.

Extrusion Process

By the process of the present invention, the polymer and wood fiber areintimately mixed by high shear mixing components with recycled materialto form a polymer wood composite wherein the polymer mixture comprises acontinuous organic phase and the wood fiber with the recycled materialsforms a discontinuous phase suspended or dispersed throughout thepolymer phase. The manufacture of the dispersed fiber phase within acontinuous polymer phase requires substantial mechanical input. Suchinput can be achieved using a variety of mixing means including,preferably, extruder mechanisms wherein the materials are mixed underconditions of high shear until the appropriate degree of wetting andintimate contact is achieved. After the materials are fully mixed, themoisture content can be controlled at a moisture removal station. Theheated composite is exposed to atmospheric pressure or reduced pressureat elevated temperature for a sufficient period of time to removemoisture resulting in a final moisture content of about 8 wt-% or less.Lastly, the polymer fiber is aligned and extruded into a useful form.

The materials fed to the extruder can comprise from about 30 to 50wt-%.of sawdust including recycled impurity along with from about 50 to70 wt-% of polyvinyl chloride polymer compositions. Preferably, about 35to 45 wt-% wood fiber or sawdust is combined with 65 to 55 wt-%polyvinyl chloride homopolymer. When using the CM-80, the preferred feedfate is approximately 600 lbs/hr polyvinyl chloride composition andapproximately 400 lbs/hr wood fiber.

The polyvinyl chloride feed is commonly in a small particulate sizewhich can take the form of flake, pellet, powder, etc. Any polymer formcan be used such that the polymer can be dry mixed with the sawdust toresult in a substantially uniform pre-mix. The wood fiber or sawdustinput can be derived from a number of plant locations including thesawdust resulting from rip or cross-grain sawing, milling of woodproducts, or the intentional commuting or fiber manufacture from wastewood scrap. Such materials can be used directly from the operationsresulting in the wood fiber by-product or the by-products can be blendedto form a blended product. Further, any wood fiber material alone, or incombination with other wood fiber materials, can be blended with wastestream by-product from the manufacturer of wood windows, as discussedabove. The wood fiber or sawdust can be combined with other fibers andrecycled in commonly available particulate handling equipment.

Polymer and wood fiber are then dry blended in appropriate proportionsprior to introduction into blending equipment. Such blending steps canoccur in separate powder handling equipment or the polymer fiber streamscan be simultaneously introduced into the mixing station at appropriatefeed ratios to ensure appropriate product composition. The later methodis preferred when using the CM-80 extruder. Mechanical mixing of thepolymer and wood begins in zone 1 of barrel 18.

In a preferred mode, the wood fiber is placed in hopper 12 and the inputrate to barrel 18 is controlled by a feed screw. The polymer isintroduced into hopper 14 and the input rate to barrel 18 is controlledby a feed screw. The pelletizing screws preferably rotate at 35 rpm±5rpm.

Each zone of the CM-80 extruder 10, adaptor 20, die 22, and screw oilhas a desired heat profile resulting in a useful product. The followingtable shows the preferred operating temperatures of zones 1-7 and thescrew oil temperature.

    ______________________________________                                        Zone         Temperature                                                      ______________________________________                                        1            235° C. ± 10° C.                                2            235° C. ± 10° C.                                3            235° C. ± 10° C.                                4            195° C. ± 10° C.                                5            180° C. ± 10° C.                                6            185° C. ± 10° C.                                7            190° C. ± 10° C.                                Screw Oil    190° C. ± 10° C.                                ______________________________________                                    

The normal barrel zone and screw oil temperatures for extrusion ofpolyvinyl chloride alone are 170° C. and 175° C., respectively. We havediscovered that higher temperatures are required to achieve the desiredphysical characteristics of the composite material. We believe that thehigher temperatures are necessary in order-to remove the moisture fromthe wood fiber and to fill the interstitial voids of the wood fiber withpolyvinyl chloride.

As the material flows through zone 1, the polyvinyl chloride begins tomelt and is blended with the wood fiber by the CM pelletizing screws. Inzone 2, the polyvinyl chloride is melted and further blended with thewood fiber. Between zones 2 and 3, moisture escaping from the wood fiberis vented from barrel 18. In zones 3 and 4, more blending and compactionof the material is accomplished resulting in the penetration ofpolyvinyl chloride into the surface pores, cracks and fissures of thewood fiber and into the interstitial voids within the wood fiber.

Once the material leaves zone 4, it passes through adaptor 20 and intodie 22, wherein the composite material stream is divided into a numberof cylindrical streams through the distal end of die 22. The die cancontain a circular distribution (6"-8" diameter) of 10 to 500 or more,preferably 20 to 250 orifices, having a cross-sectional shape leading tothe production of a regular cylindrical pellet. As the material isextruded from the head, it is cut with a double-ended knife blade at arotational speed of about 100 to 400 rpm resulting in the desired pelletlength.

While the present invention has been described in connection with thepreferred embodiment thereof, it will be understood many modificationswill be readily apparent to those skilled in the art, and thisapplication is intended to cover any adaptations or variations thereof.It is manifestly intended this invention be limited only by the claimsand equivalents thereof.

What is claimed is:
 1. A process for the manufacture of a pelletcomposite comprising thermoplastic polymer and cellulosic fiber, whichmethod comprises:(a) introducing a thermoplastic polymer composition andcellulosic fiber having cells with an interior volume into an extruder,having at least an inlet zone, a second zone, a barrel and a pellet die,at a ratio of about 35-50 parts fiber to 45-70 parts of polymer per each100 parts of the composite, the fiber having a length ranging from about1-10 mm and an aspect ratio of at least 1.8; (b) melting the polymercomposition in the barrel and blending the polymer composition withfiber under conditions of sufficient temperature, pressure and shear tocause the polymer composition to mix with the fiber to the extent thatthe polymer composition occupies greater than 50% of the interior volumeof the cells of the fiber to form a polymer fiber composite; (c)extruding the composite through the pellet die to form a thermoplasticpellet comprising a right circular cylinder having a length about 1-10mm and a radius about 1-5 mm;wherein the ratio of the density of aphysical mixture of the fiber and polymer to the density of the pelletis less than 0.8.
 2. The process of claim 1 wherein the ratio is lessthan 0.7.
 3. The process of claim 1 wherein the polymer is polyvinylchloride and the fiber is wood fiber and the pellet has a bulk densitygreater than about 0.6 gram per cm³.
 4. The process of claim 2 whereinthe bulk density of the pellet is greater than 0.7 gram per cm³.
 5. Theprocess of claim 2 wherein the bulk density of the pellet is greaterthan 0.8 gram per cm³.
 6. The process of claim 2 wherein the temperatureof the polyvinyl chloride/wood fiber composite in the extruder isgreater than the melting point of the polyvinyl chloride polymercomposition and the temperature of the polyvinyl chloride wood fiberscomposite decreases as the composite passes from the input zone to thepellet die.
 7. The process of claim 5 wherein the temperature of theinput zone is greater than 210° C. and the temperature of the pellet dieis less than 210° C. but greater than 200° C.
 8. The process of claim 5wherein the temperature of the input zone is greater than 210° C., thetemperature of at least one zone prior to the pellet die is less than195° C., while the temperature of the pellet die is greater than 195° C.9. The process of claim 1 wherein the polymer occupies greater than 65%of the interior volume of the cells of the wood fiber.
 10. The processof claim 1 wherein the polymer occupies greater than 70% of the interiorvolume of the cells of the wood fiber.